Using NASA’s Hubble Space Telescope, a team of astronomers has
spotted a star system that could have left behind a “zombie star” after
an unusually weak supernova explosion.
A supernova typically obliterates the exploding white dwarf, or dying
star. On this occasion, scientists believe this faint supernova may
have left behind a surviving portion of the dwarf star -- a sort of
zombie star.

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space Flight
Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore conducts Hubble science
operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington.

While examining Hubble images taken years before the stellar
explosion, astronomers identified a blue companion star feeding energy
to a white dwarf, a process that ignited a nuclear reaction and released
this weak supernova blast. This supernova, Type Iax, is less common
than its brighter cousin, Type Ia. Astronomers have identified more than
30 of these mini-supernovas that may leave behind a surviving white
dwarf.

“Astronomers have been searching for decades for the star systems
that produce Type Ia supernova explosions,” said scientist Saurabh Jha
of Rutgers University in Piscataway, New Jersey. “Type Ia’s are
important because they’re used to measure vast cosmic distances and the
expansion of the universe. But we have very few constraints on how any
white dwarf explodes. The similarities between Type Iax’s and normal
Type Ia’s make understanding Type Iax progenitors important, especially
because no Type Ia progenitor has been conclusively identified. This
discovery shows us one way that you can get a white dwarf explosion.”

The team’s results will appear in the Thursday, Aug. 7 edition of the journal Nature.
The weak supernova, dubbed SN 2012Z, resides in the host galaxy NGC
1309 which is 110 million light-years away. It was discovered in the
Lick Observatory Supernova Search in January 2012. Luckily, Hubble’s
Advanced Camera for Surveys also observed NGC 1309 for several years
prior the supernova outburst, which allowed scientists to compare
before-and-after images.
Curtis McCully, a graduate student at Rutgers and lead author of the
team’s paper, sharpened the Hubble pre-explosion images and noticed a
peculiar object near the location of the supernova.
“I was very surprised to see anything at the location of the
supernova. We expected the progenitor system would be too faint to see,
like in previous searches for normal Type Ia supernova progenitors. It
is exciting when nature surprises us,” McCully said.
After studying the object’s colors and comparing with computer
simulations of possible Type Iax progenitor systems, the team concluded
they were seeing the light of a star that had lost its outer hydrogen
envelope, revealing its helium core.
The team plans to use Hubble again in 2015 to observe the area,
giving time for the supernova’s light to dim enough to reveal any
possible zombie star and helium companion to confirm their hypothesis.
“Back in 2009, when we were just starting to understand this class,
we predicted these supernovae were produced by a white dwarf and helium
star binary system,” said team member Ryan Foley of the University of
Illinois at Urbana-Champaign, who helped identify Type Iax supernovae as
a new class. “There’s still a little uncertainty in this study, but it
is essentially validation of our claim.”
One possible explanation for the unusual nature of SN 2012Z is that a
game of seesaw ensued between the bigger and smaller of the star pair.
The more massive star evolved more quickly to expand and dump its
hydrogen and helium onto the smaller star. The rapidly evolving star
became a white dwarf. The smaller star bulked up, grew larger and
engulfed the white dwarf. The outer layers of this combined star were
ejected, leaving behind the white dwarf and the helium core of the
companion star. The white dwarf siphoned matter from the companion star
until it became unstable and exploded as a mini-supernova, leaving
behind a surviving zombie star.
Astronomers already have located the aftermath of another Type Iax
supernova blast. Images were taken with Hubble in January 2013 of
supernova 2008ha, located 69 million light-years away in the galaxy UGC
12682, in more than four years after it exploded. The images show an
object in the area of the supernova that could be the zombie star or the
companion. The findings will be published in The Astrophysical Journal.

“SN 2012Z is one of the more powerful Type Iax supernovae and SN
2008ha is one of the weakest of the class, showing that Type Iax systems
are very diverse,” explained Foley, lead author of the paper on SN
2008ha. “And perhaps that diversity is related to how each of these
stars explodes. Because these supernovae don’t destroy the white dwarf
completely, we surmise that some of these explosions eject a little bit
and some eject a whole lot.”
The astronomers hope their new findings will spur the development of
improved models for these white dwarf explosions and a more complete
understanding of the relationship between Type Iax and normal Type Ia
supernovae and their corresponding star systems.